Ceramic discharge vessels, typically made of Al.sub.2 O.sub.3 ceramic, with or without additives, and using a metal-halide fill, have improved color rendition of light outputs. Typical ratings of such lamps are between 100 to 250 W.
The seal of the feedthrough of conductors leading to the electrode in the interior of the vessel presents a major problem. Frequently, the feedthrough is made of niobium and, often, it is fitted into a plug of ceramic and vacuum-tightly sealed therein by a glass melt, or ceramic sealing material--see, for example, Published European Application 0 472 100 A3, Weske et al, assigned to the assignee of the present application. The metal halides in the fill have a highly corrosive effect on both the niobium feedthrough and the glass melt, so that the useful life of such lamps has been short. A large number of different compositions for glass melts, glass frits, also known as glass solders, have been tested. U.S. Pat. No. 4,122,047, Meden-Piesslinger et al, for example, describes a glass melt which has at least two oxides selected from SiO.sub.2, Al.sub.2 O.sub.3 and B.sub.2 O.sub.3, and at least one of the oxides of yttrium or lanthanun, or other rare earths. U.S. Pat. No. 5,099,174, Coxon et al, describes a glass sealing composition having a very high content of SiO.sub. 2 (45-50%, by weight), balance Al.sub.2 O.sub.3 and MgO. All the glass compositions having a relatively high content of SiO.sub.2, ranging between 20-50% by weight, however, are susceptible to reaction with halides.
Other glass sealing compositions having a very low content of SiO.sub.2, that is, between 0-20% by weight, have been described in U.S. Pat. No. 4,501,799, Driessen et al, and U.S. Pat. No. 4,980,236, Oomen et al. These compositions use Al.sub.2 O.sub.3, Sc.sub.2 O.sub.3 and TiO.sub.2, as well as rare earth oxides and alkaline earth oxides, and have very high melting points, that is, 1500.degree.-1700.degree. C. They have, however, unsuitable solidification characteristics and form imperfect seals, subject to leakage.